1. Field of the Invention
The present invention relates to a method and apparatus suitable to divide a semiconductor wafer into a large number of semiconductor devices by laser beam irradiation. In particular the invention relates to a technique for laser beam irradiation on the rear surface side of a semiconductor wafer not formed with devices.
2. Description of the Related Art
A wafer formed with a large number of semiconductor devices on the front surface thereof is thinned to a required thickness by grinding the rear surface thereof. Thereafter, the wafer is stuck to a dicing tape with a dicing frame and cut along cut lines called streets between the semiconductor devices into individual semiconductor devices. As a wafer dividing apparatus, a dicing apparatus is generally used in which a hub blade with a thickness of approximately 10 to 30 μm cuts streets for cutting the wafer while rotating at a high-speed of 30,000 rpm or more. However, a laser processing apparatus is also used other than the dicing apparatus. The laser processing apparatus emits laser beams along streets and employs a technique for directly cutting a wafer by laser beam irradiation or for irradiating the inside of a wafer with a laser beam to form a modifying layer and dividedly cutting the modified portion.
In the case of dividing a wafer using the laser processing apparatus, if a laser beam is directed to a street from the front surface side formed with devices, a problem is likely to occur as below. A fused material called debris splashes or flows and adheres to the surface of a device or a laser beam impinges on a TEG which is a metal test pattern formed on the front surface side of a street. To eliminate such a problem, a technique for directing a laser beam to a wafer from the rear surface side thereof is proposed in Japanese Patent Laid-open No. 2004-22936.
It is necessary to recognize the positions of streets in order to emit laser beams along the streets. The recognition method includes the following example. Infrared light is directed to a wafer from the rear surface thereof. Light reflected by the wafer rear surface is received by a microscope installed on the rear surface side of the wafer to provide an image. Streets are recognized from the pattern image of the wafer front surface thus obtained. However, in this method, infrared light is diffusely reflected by a wafer such as the so-called epitaxial wafer doped with boron or the like or by a wafer that is variously doped or thermally-treated by the previous process. It is sometimes difficult to obtain the accurate pattern image of the front surface, which consequently produces a problem in that the wafer cannot accurately be cut along the streets.